1. Field of the Invention
The invention relates to a physical method for stabilizing the electrolyte content of latex compositions particularly the iron ion content of latex autodeposition baths.
2. Statement of the Related Art
The autodeposition process and autodeposition baths associated therewith (best known under the "Autophoretic" trademark of Amchem Products, Inc, Ambler, Pa. U.S.A.--a subsidiary of Henkel KGaA, Duesseldorf, Germany) is a process for pretreating and coating metal surfaces, particularly steel and iron surfaces, in which an acidic dispersion of an organic resin in water (i.e. a latex) containing pigments and auxiliaries is brought into contact with metal surfaces and, providing certain conditions are maintained, an organic coating is produced on those surfaces by a chemical reaction involving the substrate and the coating material. By virtue of its many advantages, this process is being used to an increasing extent in the metal-processing industry to impart effective protection against corrosion to metal parts, particularly to awkwardly situated metal parts. The autodeposition process (c.f. R. Morlock, "Jahrbuch Oberflachentechnik" 36, 327 (1980)) essentially comprises contacting metal surfaces, for example steel or iron surfaces, with an acidic aqueous dispersion containing resin-wetting agent or internally stabilized resin micelles. Under the corrosive attack of the acid on the iron surface, iron- (II) (i.e. ferrous) ions pass into solution, interacting with the negatively charged micelles and thus at least partly breaking down the negative charge by which the dispersion is stabilized. The dispersion coagulates at the dispersion/metal surface interface, resulting in the formation of a film adhering to the surface.
Unfortunately, more iron is generally dissolved by the corrosive reaction than is required for the equilibrium of the desired coating reaction. The excess iron ions accumulate in the coating bath and destabilize the latex. If certain limits are exceeded, the contents of the bath coagulate and the desired coating reaction can no longer take place.
Several attempts have been made to prevent this accumulation of excess ions and to stabilize the autodeposition bath at an optimum iron ion level. Thus, it is proposed in "Plaste und Kautschuk" 27:349 (1980) to add precipitants, particularly Ca(OH).sub.2, to the coating bath in order to precipitate the iron, oxidized to ferric by air or oxidizing agents, in the form of iron hydroxide. However, the disadvantage of adding calcium hydroxide to the coating bath is that other foreign ions (i.e. calcium) are introduced into the dispersion, in addition to which the acidity of the coating baths is undesirably displaced towards the alkaline range.
U.S. Pat. No. 3,791,431 discloses a process for autodeposition coating metal surfaces in which excess ferric iron ions are precipitated with phosphoric acid or alkali fluorides as phosphate or as complex fluorides or are complexed by additions of ethylene diamine tetraacetic acid (EDTA) or organic mono- or oligocarboxylic acids (citric acid, gluconic acid, tartaric acid or lactic acid) and thus removed from the solution or converted into a form which no longer gives rise to destabilization of the dispersions in the coating baths. The disadvantage of the additions described in the above patent is that certain pH-ranges which do not exactly coincide with the pH-value of the process have to be maintained for efficient precipitation or chelation. In addition, the deposits or complexes remain in the bath and partially redissolve when the pH-value is adjusted by the addition of hydrofluoric acid. Accordingly, the inorganic ions remaining in the bath solution continue to contribute towards destabilization of the dispersion. These disadvantages can only be partly offset by elaborate filtration processes.
U.S. Pat. No. 3,839,097 describes a process for stabilizing autodeposition baths, in which the baths, after the addition of surfactants, are passed over ion exchangers which are capable of removing the excess quantities of metal ions detrimental to the stability of the dispersions from the baths. This process is also apparatus-intensive, in addition to which the aqueous dispersions are inevitably impaired by pump-recirculation through the exchanger columns, so that a deterioration in the quality of the bath results.
Even an addition of dispersants and of a quantity of resin exceeding the concentration required for the formation of uninterrupted resin layers on the metal surface to the dispersion, as described in U.S. Pat. No. 3,936,546, could not satisfactorily eliminate the above disadvantages.
U.S. Pat. No. 4,414,350 discloses complexing the iron salts present in excess in an autodeposition bath, with at least one carboxylic acid such as acetic, succinic, acrylic, and the like. This process resulted in a sludge-laden bath.
More recently, as disclosed in copending U.S. patent application No. 06/725,470, which is assigned to a commonly owned company and whose disclosures relating to iron salt build-up are incorporated herein by reference, a method for bath stabilization (i.e. iron ion control) has been proposed in which one continuously or intermittently discards a predetermined volume of the bath, which volume is replaced by an equal volume of water, the discard rate being chosen so as to maintain a substantially constant concentration of iron salts in the bath. Independently of this, the other ingredients of the bath (latex, pigment, acid, etc.) are replenished at a rate sufficient to maintain each at its original concentration. The results of this method have been excellent with regard to both performance and stability. However, a considerable amount of latex and pigment is irretrievably lost, which increases the cost of the autodeposition process.